1. Field of the Invention
This invention relates to a track deviation detecting apparatus for use with a device for optically making a record on a recording medium having guide grooves.
2. Description of the Prior Art
In order to enable a reading spot of light projected on a recording medium, such as an optical disk, video disk or digital audio disk, to accurately follow the tracks, it has been practiced to detect the relative position of the track and reading spot of light. As a reliable method for such detection, there has been proposed a twin-spot method which separates the light emitted from a light source into at least three beams of light (Japanese Patent Laying-Open No. 50954/1974, priority claimed on Dutch Patent Application No. 7206378).
FIG. 1 is a view showing the mechanism of such a conventional optical information reading device. In the figure, the light emitted from a light source 1 is separated into three beams of light by diffraction grating 2. The separated beams pass through a lens 3 and a half mirror 4 and is reflected by a rotary mirror 5 to travel to a condenser 6. The three beams of light are concentrated on a recording medium 7 as three spots of light by the condenser 6.
The three concentrated spots of light are reflected from the recording medium 7 and concentrated again by the condenser 6 and reflected by the rotary mirror 5 and half mirror 4. Thus, the reflected light rays from the three spots of light on the recording medium are guided to optical detectors 8, 9 and 10 corresponding to the respective spots of light.
The disposition of the three spots of light on the recording medium is shown in FIG. 2. Tracks 11 have upper and lower edges 12 and 13 and are formed on the recording medium 7 so that they extend parallel to the direction of travel of the recording medium. A spot of light A is formed by the diffracted light of the zeroth order from the diffraction grating; that is, it is the spot of light formed by the beam guided along an optical path 30a in FIG. 1 and concentrated on the recording medium 7 and serves to read the information recorded on the track. An auxiliary spot B1 is formed by the diffracted light of the plus first order from the diffraction grating; that is, it is the spot of light formed by the beam guided along an optical path 30b 1 in FIG. 1 and concentrated on the recording medium 7, and is concentrated on the lower edge 13 of the same track as that on which the spot of light A is concentrated. An auxiliary spot of light B2 is formed by the diffracted light of the minus first order from the diffraction grating; that is, it is the spot of light formed by the beam guided along an optical path 30b 2 in FIG. 1 and concentrated on the recording medium 7, and is concentrated on the upper edge 12 of the same track as that on which the spot of light A is concentrated. The centers of the spot of light A and auxiliary spots B1 and B2 are located on the same line, and the spacing between the spot of light A and the auxiliary spot B1 and the spacing between the spot of light A and the auxiliary spot B2 are equal and constant. Accordingly, where the spot of light A is located at the center of the track 11, an area of the auxiliary spot B1 to be occupied on the track 11 is equal to an area of the auxiliary spot B2 to be occupied on the track 11.
Reflectivity of the track 11 differs from that of the portion on the recording medium 7 where the track 11 is not formed.
The spot of light A and auxiliary spots B1 and B2 are reflected from the recording medium 7 and led to the high frequency optical detectors 9, 10 and 8 corresponding thereto.
The operation of the track deviation detecting mechanism arranged in the manner described above will now be described with reference to FIGS. 1 and 2.
When the projected position of the spot of light A on the track is moved, the auxiliary spots B1 and B2 are moved in the same direction and over the same distance as the spot of light A. Where the spot of light A is located at the center of the track 11 to be read in FIG. 2, the optical detectors 10 and 8 to which the auxiliary spots B1 and B2 are led receive an equal quantity of light since the area of the auxiliary spot B1 to be occupied on the track 11 is equal to the area of the auxiliary spot B2 to be occupied on the track 11.
Where the spot of light A is deviated from the center of the track 11 in FIG. 2, the area of the auxiliary spot B1 occupying the track 11 becomes different from the area of the auxiliary spot B2 occupying the track 11. The reflectivity of the track 11 differs from that of the portion on the recording medium 7 where the track 11 is not formed, as described before. Therefore, the auxiliary spots B1 and B2 are reflected from the recording medium 7 to the optical detectors 10 and 8 corresponding thereto with different quantities of light. Thus, the intensities of the light beams incident on the optical detectors 10 and 8 differ from each other and hence the optical detectors 10 and 8 deliver different electric output signals.
In FIG. 1, the optical detectors 10 and 8 have a differential amplifier 14 connected thereto, so that if the electric output signals from the optical detectors 10 and 8 are amplified by the differential amplifier 14, it is possible to detect the amount and direction of the deviation of the spot of light A from the track to be read.
According to the method described above, there is no inconvenience when a recording medium having information recorded thereon is to be reproduced. However, when information is to be recorded on a recording medium which has not been recorded, the following drawback interferes with normal operation.
This drawback will now be described with reference to FIG. 3. FIG. 3 shows a guide groove 15 called a pregroove on a recording medium, and a spot of light A and auxiliary spots B1 and B2, illustrating their relative position, in accordance with a conventional device. The spot of light A serves to write records. In FIG. 3, the guide groove 15 on the recording medium moves in the direction of arrow X, and the spot of light A forms record pits 16 on the guide groove along the latter. The auxiliary spots B1 and B2 are respectively positioned on the lower and upper edges 13 and 12 of the guide groove 15, the relative position of the spot of light A and auxiliary spots B1 and B2 being the same as that shown in FIG. 2.
In FIG. 3, since the guide groove 15 moves in the direction of arrow X, the auxiliary spot B2 passes over the record pits 16 formed by the spot of light A. On the other hand, the auxiliary spot B1 is always projected on the unrecorded portion where the record pits 16 are not formed. The reflectivity of the record pit 16 differs from that of the unrecorded portion where the record pits 16 are not formed. Thus, even if the writing light spot A is positioned at the center of the guide groove, that is, even if it is in the normal writing position, the quantity of light reflected from the auxiliary spot B2 influenced by the record pit 16 differs from the quantity of light reflected from the auxiliary spot B1 not influenced by the record pit 16.
Consequently, the optical detectors 10 and 8 of FIG. 1 respectively associated with the auxiliary spots B1 and B2 will have their outputs influenced. That is, despite the fact that the amount of deviation from the track is zero, the output of the differential amplifier 14 connected to the optical detectors 10 and 8 does not become zero. Further, in this case, since the output from the differential amplifier 14 depends also on the density of the record pits 16, i.e., the spacing between the record pits, and on the size of the record pits, electrical compensation of balance is difficult. The method described above, therefore, has the drawback that it cannot be applied to the optical information recording device.